1. Field of the invention
The present invention relates to a braking system for applying braking force to wheels of a vehicle, and more particularly to the braking system having a pressure generator for generating a hydraulic braking pressure to wheel brake cylinders in response to operation of a brake pedal, and a stroke simulator for providing a dummy braking load to the brake pedal.
2. Description of the Related Arts
With respect to a vehicle braking system, an apparatus called "brake by wire" has been noted recently. The brake by wire apparatus is provided with a pressure generator for generating a hydraulic braking pressure independent of a conventional master cylinder, and adapted to control a braking pressure control valve device in response to operation of the brake pedal to supply the hydraulic braking pressure from the pressure generator to the wheel brake cylinders, and supply the hydraulic braking pressure output from the master cylinder to a stroke simulator, as shown in Japanese Patent Laid-open Publication No. 2-49943, for example. The publication discloses a braking apparatus having a servo pressure generating device, wherein a device associated with the master cylinder for providing a brake feeling, which corresponds to the stroke simulator, is connected to a brake passage, which is opened or closed by an electromagnetic valve device. The valve device is turned on when a servo braking system is normal, and turned off when it is failed. The braking apparatus has a pressure control valve with a plurality of positions to be selected.
According to the braking apparatus as disclosed in that publication, if the pressure control valve or the like is failed, for example, it is necessary for a vehicle driver to apply a relatively large force to the brake pedal to obtain a sufficient braking force. In order to solve this problem, it can be proposed to provide a conventional booster such as a hydraulic booster or vacuum booster. In this case, however, if either one of those boosters is installed in that braking apparatus, the hydraulic braking pressure applied to the stroke simulator will be large even in the normal state. In order to overcome this problem, therefore, it is necessary to install a spring with a large spring constant in the stroke simulator. As a result, the stroke simulator is made large in size, and high in cost.